High PPI (Pixels Per Inch) products have currently become a main developing trend for display devices with a small or medium size. A top-gate structure is generally adopted in products, for purpose of realizing a TFT (thin film transistor) with improved characteristics to meet the charging demand of high PPI products.
However, the process for the top-gate structure is generally complicated, and there are numerous masking processes. Therefore, the cost and yield have become the concerns of the panel manufacturers. To prevent the strong light from a backlight from directly irradiating a back channel which would lead to leakage current, a light shielding layer is generally formed prior to the semiconductor layer. The light shielding layer is usually made from metal Mo, and a pattern is formed by a separate masking process.
In some products, the light shielding layer is omitted to decrease the number of masking processes and thus reduce the cost. Instead, the performance of these products is ensured by improving the process capacity of other portions in TFT and optimizing the pixel design. However, this solution is abandoned soon during upgrade of products, because a large leakage current is not acceptable for a high performance product. Thus, there is still a need for arranging the light shielding layer in the top gate structure.